The membrane separation method, which employs the difference in gas permeability with respect to a membrane, is known as a method for separating a gas mixture including at least two different gas species into the respective gases. With this method, at least either a highly-pure high-permeability gas or a highly-pure low-permeability gas, which are the target gases, can be obtained by recovering the permeate gas and/or the retentate gas. The permeability—which is the volume of permeation, through a membrane, of each gas included in the gas mixture per unit membrane area, per unit time, per unit partial pressure difference—can be expressed as P′ (unit: ×10−5 cm3 (STP)/cm2·sec·cmHg). The gas selectivity of the membrane can be expressed as the ratio of the permeability of the high-permeability gas to the permeability of the low-permeability gas (i.e., high-permeability gas's permeability/low-permeability gas's permeability).
Generally speaking, a gas separation membrane with high gas selectivity has a low gas permeability, and conversely, a membrane with a high gas permeability has a low gas selectivity. Thus, in cases of recovering a low-permeability gas from a gas mixture by using a single-stage gas separation membrane, the use of a membrane with high gas selectivity increases the recovery rate if the purity of the gas to be recovered is constant. In this case, however, the permeability is low, and thus, it is necessary to increase the membrane area or increase operation pressure. On the other hand, a membrane having a high permeability does not require an increase in membrane area or an increase in operation pressure, but because its gas selectivity is low, the recovery rate is reduced.
In general, a gas separation membrane is employed in the form of a gas separation membrane module wherein the gas separation membrane having gas permselectivity is housed inside a container which includes at least a gas inlet port, a permeate gas discharge port, and a retentate gas discharge port. The gas separation membrane is installed inside the container in a manner so as to separate the space on the gas feed side and the space on the gas permeation side. In a gas separation system, in general, a plurality of such gas separation membrane modules are combined in parallel and are used as a gas separation membrane unit in order to obtain a predetermined membrane area. The plurality of gas separation membrane modules constituting the gas separation membrane unit share the gas inlet port, the retentate gas discharge port, and the permeate gas discharge port, and thus, the gas separation membrane unit functions substantially as a large gas separation membrane module.
In order to recover the target low-permeability gas with high purity and a high recovery rate, a known method employs a system that includes multiple stages of the aforementioned gas separation membrane units. Examples of such multi-stage gas separation systems include: a system in which retentate gas of the first stage obtained by enriching a low-permeability gas is further separated in order to increase purity; and a system in which low-permeability gas included in the permeate gas of the first stage is recovered in order to improve the recovery rate.
As regards multi-stage gas separation membrane units, Patent Literature 1, for example, proposes a gas separation method involving three gas separation membrane units, which correspond to separation stages (1) to (3). In the gas separation method disclosed in Patent Literature 1, a gas mixture to be separated is fed to a separation stage (1) (first gas separation membrane unit). Retentate gas discharged from the separation stage (1) is supplied to a separation stage (2) (second gas separation membrane unit), and permeate gas discharged from the first gas separation membrane unit is fed to a separation stage (3) (third gas separation membrane unit). Further, permeate gas discharged from the second gas separation membrane unit and also retentate gas discharged from the third gas separation membrane unit are returned to the first gas separation membrane unit (cf. FIG. 11 of Patent Literature 1). Claim 1 of Patent Literature 1 describes that “preferably, in all of the three membrane separation stage (1) to (3), the mixed gas selectivity is at least 30”, and the Example described therein employs three separation membrane units having the same, high gas selectivity.